The present invention related to operational amplifiers, and more particularly to an improved amplifier structure which consumes no electric current and provides a stable output when not working.
Following rapid development of integrated circuit technology, advanced ICs, such as micro-controllers, digital-to-analog converters, memory devices, and etc., are made having a minimized dimension with sophisticated operation capability. Operational amplifiers are used in a variety of IC chips. FIG. 1 illustrates a conventional operational amplifier. This amplifier structure is composed of CMOS (complementary metal-oxide semiconductor) comprising a differential amplifier 10, a reverse output buffer 20, a non-inverting terminal IP, an inverting terminal IN, a bias voltage input terminal AVB, and an output terminal OI. The differential amplifier 10 is composed of semiconductor switches M1xcx9cM5, and is connected to the non-inverting terminal IP and the inverting terminal IN. The reverse output buffer 20 is comprised of two semiconductor switches, namely, the reverse switch M6 and the forward switch M7 connected between a reference voltage and a grounding terminal GND. The output terminal O1 is connected between the semiconductor switches M6 and M7 of the reverse output buffer 20. An input bias voltage signal is provided through the bias voltage input buffer 20 to electrically connect the forward switch M7, so as to achieve proportional output. This structure of this operational amplifier is commonly used in mobile electronic apparatus such as mobile telephones, notebook computers, and personal digital assistant. However, this amplifier structure is not satisfactory because it consumes to much electric current when not working.
It is the main object of the present invention to provide an operational amplifier, which consumes no power AND provides a stable output signal through its output terminal when it is not working. The reverse output buffer according to the present invention comprises a second reverse switch and a second forward switch. The second reverse switch is connected is connected in series between the first reverse switch and the reference voltage. The second forward switch is connected is connected between the output terminal and the grounding terminal GND. A bias voltage provided from a second bias voltage input terminal of the operational amplifier electrically connects the second reverse switch and the second forward switch. When the second bias voltage input terminal is at low potential and the first bias voltage input terminal is at high potential, the second reverse is on, and the second forward switch is off, thereby causing the operational amplifier to function normally. When the s second bias voltage input terminal is at high potential and first bias voltage input terminal is at low potential, the second reverse switch is off, and the second forward switch is on, thereby causing the operation amplifier to consume no electricity, and the output terminal to provide a stable low voltage output.